Manganese activated magnesium-lithium alumino-gallate luminescent material

ABSTRACT

Manganese-activated lithium gallate, magnesium-lithium gallate, and magnesium-lithium alumino-gallate phosphors emit in the green region when excited by cathode rays and short-wavelength ultraviolet radiation.

llnited States Patent Datta [451 Jan. 18, 1972 [54] MANGANESE ACTIVATED 3,499,843 3/1970 Brown, Jr. et al ..2s2/301.4

MAGNESIUM-LITHIUM ALUMINO- OTHER PUBLICATIONS GALLATE LUMINESCENT MATERIAL App1.No.: 819,919

U.S. Cl. ..252/30l.4 R Int. Cl. ..C09k 1/04, C09k 1/68 Field of Search ..252/301.4

References Cited UNITED STATES PATENTS Brown, Jr. ..252/301.4

Primary Examiner-Robert D. Edmonds Attorney-Henry P. Truesdell, Joseph B. Fonnan, Frank L. Neuhauser, Oscar B. Waddell and John F. McDevitt [5 7] ABSTRACT Manganese-activated lithium gallate, magnesium-lithium gallate, and magnesium-lithium alumino-gallate phosphors emit in the green region when excited by cathode rays and shortwavelength ultraviolet radiation.

3 Claims, 4 Drawing Figures MANGANESE ACTIVATED MAGNESIUM-LITHIUM ALUMINO-GALLATE LIJMINESCENT MATERIAL BACKGROUND OF THE INVENTION This invention relates to luminescent materials. More particularly, the invention relates to a group of manganese-activated lithium gallate phosphors.

Magnesium gallate-type phosphors have recently been disclosed for their possible application in fluorescent lamps used in xerographic copying machines. Brown, in US. Pat. No. 3,407,325, and in an article appearing in theJournal of the Electrochemical Society, March, 1967, pp. 33-37, observed an efficient luminescence in manganese activated magnesium gallate. Wanmaker et al., in an article appearing in Philips Res. Repts, 22, pp. 304-308, 1967, modified the formulation suggested by Brown by substituting a small amount of aluminum for gallium and observed a shift in the excitation band to a shorter wavelength. The luminescence quenching temperature of the luminescence of the aluminum-incorporated phosphor was found to be higher than that of the manganeseactivated magnesium gallate. The composition of the matrix suggested by Brown was contained in the system MgOGa O whereas the composition of the phosphor proposed by Wanmaker et al. was part of the ternary system MgOGa O B1 In the present invention, the composition of the phosphor has been modified by incorporating lithia (Li O) with proper charge compensation in the matrix. Such modification improves the luminescent properties of the phosphor such as room temperature brightness and high-temperature brightness. Thus, the incorporation of lithia in the phosphor of the present invention significantly increases the brightness of the phosphor and, further, the quenching temperature of the luminescence is raised. Further, the excitation bands become broader and the firing of the material can be done at a lower temperature than that suggested by the prior art without losing any brightness of the phosphor.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a luminescent material containing lithium having improved brightness at room temperature and at elevated temperatures.

Another specific object is to provide a fluorescent material wherein the quenching temperature of the luminescence is raised.

Another object is to broaden the excitation bands in rela' tion to the aluminum substituted magnesium gallate, and a further object is to provide a phosphor that can be synthesized at a lower temperature without loss of brightness.

Briefly stated, the invention relates to a luminescent material consisting essentially of a crystalline compound of lithium gallate, optionally containing magnesium, and optionally containing aluminum, and having manganese incorporated integrally into the matrix as an activator, said compound having a crystal structure selected from the group consisting of a spinel-type structure and a simple cubic structure, and said compound containing between 0.001 and 0.5 moles of lithium per mole of the compound. More specifically, compounds of the invention have the following formula:

Mg, ,,Li Ga Al O :xMn where x has a value of from l to 0.7, y hasa value of from 0.001 to 0.5, 2 has a value of from 0 to 1.0, and x has a value of froin 0.00l to 0.07. As is commonly done in phosphor terminology, the activator is identified herein after a colon, as

in :0. 1 Mn, and this indicates that the activator is incorpo-.

rated in the lattice of the phosphor matrix.

In the above formula, preferablyx is fidfri058tdl0, y is from 0.001 to 0.06, and z is from 0 to 0.6. The preferred range of aluminum content is achieved when 2 is from 0.05 to 0.4. Manganese is generally divalent in these phosphors and preferably has a concentration of x from 0.005 to 0.04. When x is 1.0 and y is 0.5, there is no magnesium in the composition, and the phosphor is lithium gallate activated by manganese and optionally containing aluminum.

Those parts of the present invention which are considered to be new are set forth in detail in the claims appended hereto. The invention, however, may be better understood and further objects and advantage thereof appreciated from a consideration of the drawings and detailed description.

DRAWINGS AND DETAILED DESCRIPTION FIG. 1 is a diagram showing the subsolidus phaseequilibrium relationships in the ternary system MgO-Ga,O,-Li,0 at 1 ,400 C.

FIG. 2 is a graph showing emission spectra of phosphors with and without Li O.

FIG. 3 is a graph depicting improved brightness due to the incorporation of Li O in two different matrices, one with and the other without aluminum.

FIG. 4 is a graph of the relative brightness of phosphor compositions at varying temperatures.

The phosphors of this invention exhibit a spine] structure or its crystalline solution. The prior art compositions were contained in the systems lin o-c21 0, of the above-cited Brown patent and MgO-Ga O -Al O of the above-cited Wanmaker at al. article. The present system incorporates Li O in the spinel lattice, and the composition of the matrix of the phosphor can be expressed in the terms of the quaternary system M o-Li,0-Ga,o,-Ai,0,. However, since the proportion of M 0, is very small and for the sake of simplicity, the composition is diagrammatically shown in FIG. 1 as involving the ternary system MgO-Ga O -Li- O. In addition to the quaternary, the invention also includes that ternary system itself without any A1 0,. It can be further simplified by saying that the matrix is essentially a crystalline solution between Mg- Ga O and LiGa O with or without excess of Ga ,Al,O (z= 0 to I) over the stoichiometric proportion. Pure LiGa O (Li,, ,,Ga -,O. when Fl, y=0.5, z=0) has two polymorphs: the low-temperature simple cubic phase undergoes a very fast, reversible transition at 1,138 fl" C. to the high-temperature spinel phase. Both of these polymorphs have significant luminescence and show extensive solid solution with MgGa o, and Ga ,Al,O The high-temperature phase cannot be quenched rapidly enough to be retained to room temperature even in ordinary laboratory size samples (about 5-l0 g.) when y in the above equation is about 0.3 or more, as indicated by the dotted line boundaries at the right of the spinel phase in FIG. 1. Both forms of LiGa o and its crystalline solution with MgGa O show luminescence.

FIG. 1 shows the spinel forming compositions of the system MgO-Ga O -Li O at l,400 C. Any composition in this ternary system indicated by any point in the shaded area will, on equilibration at 1,400 C., result in a single spinel-type phase having the same crystal structure as a true spinel composition. In other words, any composition lying within the area ABCD will form a spinel when equilibrated at l,400 C. However, as the temperature of equilibration increases, the spinel-forming area ABCD expands. When the temperature of equilibration is lower, the area ABCD shrinks. According to the present invention, the phosphor can be synthesized within wide composition and temperature ranges. However, the composition and the firing temperature should be so selected that the final luminescent material is of spine] or simple-cubic structure.

Referring now to FIG. 2, there is shown the emission spectra of:

l. LiGa 0 :0.0lMn

Manganese-activated magnesium-lithium alumino-gallate (spinel) when excited by cathode rays or ultraviolet radiation shows narrow band emission spectrum extending from 465 to 585 nanometers (nm.) with a peak of 505 nm. As referred to above, LiGa O itself, and compositions of the invention that are primarily LiGa O have two polymorphs: the lowtemperature simple cubic form, and the high-temperature spinel fonn. The low-temperature form or its crystalline solutions activated with Mn show narrow-band emission peaking at 5 1 2JJH 1-. 5 dr ath dqrays Short avel h l le radiation.

The unactivated gallates of the system MgO-Ga O -Li O- A1 such as Mg ,,I.i,, ,,Ga, AL ,0 and LiGa are white body colored, and show blue luminescence (emission maxima at about 405 nm.) under cathode rays and ultraviolet radiation. When a small amount of Mn is incorporated in the gallate lattice (spinel) the material acquires a white to pale green body color and responds very well to cathode rays and ultraviolet radiations especially radiations with wavelengths ranging from 200-280 nm. The nature of the emission and the relative responses are functions of composition.

FIG. 3 shows improved brightness due to the incorporation of varying amounts of Li O in Mg Ga, Al O :0.0lMn. The relative brightness is shown in table I below. FIG. 3 also shows the effect on brightness OF INCORPORATION of LiGa O in the Mg ,,Ga O :0.01Mn lattice, as given in table 11 below. To show the beneficial effects of incorporating Li O in the matrices, two previously disclosed phosphor compositions, namely Mg Ga, Al O,,:0.01Mn and Mg Ga O,:0.01Mn, are compared with phosphors of the invention.

FIG. 4 depicts relative brightness as temperature is increased of:

l. Mg Ga O :0.0 1 Mn of the prior art,

2. Mg ,,Ga, Al O,:0.0lMn of the prior art, and

3. M g Li Ga, Al O :0.0lMn of the invention.

It has been observed that manganese-activated magnesium gallate shows a sharp decrease in photoluminescent brightness with increase in temperature of the phosphor. When small amounts of Al O are incorporated in the MgGa O :Mn lattice, it is observed that the quenching temperature of the luminescence is higher than that of magnesium gallate Mg- Ga O.,:Mn. According to the present invention, incorporation of Li O (with proper charge compensation) in the matrix raises the quenching temperature to even a higher value. Thus, when excited under 2,537 A. radiation, Mg Ga O, :0.01Mn shows 50 percent of its room temperature brightness when the sample is heated to about 140 C., whereas Mg ,,Ga, l O :0.01Mn shows 50 percent of its room temperature brightness at 185 C. A preferred embodiment of this invention (Mg Li Ga, Al O :0.0lMn) loses 50 percent of its room temperatures brightness only at temperatures well above 250C., as shown in FIG. 4.

The following tables I and I] show the improved relative brightness of phosphors of the invention, along with peak emission wavelengths, due to the incorporation of varying amounts of Li O, represented in table I by y, in the matrix.

TABLE I.-EMISSION WITH VARYING Li CONTENT 1 Simple cubic structure.

TABLE IL-EMISSION WITH VARYING SPINEL PHASE Relative brightness Peak Mole percent 253.7 nm. Emission Composition of the spinal phase. LiGmOi Mn excitation (nm.)

MgGazOuO. 1 Mn 0 85 505 MgGHIOlIO-Ol Mn and LiGa O5:0.01 Mn 1 87 505 Same as above 2 98 505 D0 5 105 50 D0 10 105 605 Do 100 505 Do 92 505 Do 506 Do 58 l 506 Do 48 1 508 LlG850520-0 lMn 28 512 1 Simple cubic phase.

The fluorescent materials can generally be prepared by a three step method. Since the final crystallographic structure of the matrix is a function of temperature and composition, the highest temperature of firings can be selected in the range of 900l400 C. or higher to obtain the spine] or simple-cubic structure.

The preparation of the spinel form of the phosphor involves a three-step solid-state firing process. MgCO Li-,CO;,, Ga,0,, A1 0 and MnCO, were weighed and mixed in appropriate proportions. The mixed sample was heated at a lower temperature such as l,000 C. for about 2 hours to minimize lithium voiatilization, followed by a second heat treatment at 1,400 C. for 8 hours to form the compound. The first two heat treatments were carried out in air, and the samples were contained in platinum containers. The third and the final heat treatment was carried out in a very mild reducing atmosphere at l,200 C. for about 2 hours and the samples were contained in silica boats. During this reduction, Mn was reduced to Mn with little reduction ofGa Specific examples of the procedures used in the preparation of the phosphor composition of this invention are given as follows:

EXAMPLE I 0.923 g. of Li CO 11.715 g. ofGa O 0.061 g. of MnCO (58.01% MnO by weight) These ingredients were mixed and fired at l,000 C. for 2 hours, followed by regrinding and a second firing at 1,400 C. for 8 hours. The sample was cooled, reground and finally reduced at l,200 C. for 1 15 hours under very mild reducing conditions (a mixture of hydrogen and nitrogen gas passed through the furnace). Under such conditions any quantity of a higher valent form of Mn is reduced to Mn The finished phosphor, LiGa O :0.01Mn responds very well to cathode rays and short-wavelength ultraviolet radiations and emits a narrow-band spectrum with a peak at 5 12 nm.

EXAMPLE II To show that lithia (Li O)-incorporated samples fired at lower temperature produce the same or higher brightness as samples without Li- O fired at 1,400 C., the following samples with two different compositions were prepared. SAMPLE 1 4.302 g. ofMgCO (42.5% MgO by weight) 9.372 g. ofGa 0 0.061 g. ofMnCO (58.01% MnO by Weight) The above ingredients were mixed together and fired at l,000 C. for 2 hours. The product was divided into two parts. Part A was fired at l,400 C. for 8 hours in air, then in a reducing atmosphere at l,200 C. for 1% hours. Part B was fired at 1,300 C. for 8 hours in air, then in a reducing atmosphere at l,200 C. for 1% hours. Thus, two differently heat-treated (1,400 C., 1,300 C.) samples with the composition Mgo. Ga OBY4:0.0 1 Mn were obtained.

SAMPLE 2 4.2542 g. of MgCO (42.5% MgO by weight) 9.395 g. of Ga O 0.009 g. of Li CO 0.061 g. of MnCO; (58.01 MnO by weight) The ingredients mixed together underwent the same heat treatment as sample 1, and two samples of the composition Mg Li O :0.01Mn fired at 1,300 C. and 1,400 C. were obtained. The following brightness data were derived, showing improvements over the materials of the above-cited Brown patent:

TABLE III Brightness vs. Firing Temperature Highest Firing Relative Composition Temp. C. uv-brightness M ,,oa,0,:0.o|Mn 1,400 100 Mg Ga O ODlMn 1,300 89.7 M nn oa o ooiMn 1,300 101.2 Mg ,,,,Li Ga, O :0.0 lMn 1.400 118 EXAMPLE 111 thereof, it is to be understood that this invention is not limited to the specific embodiments discussed except as defined in the appended claims.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. A luminescent material consisting essentially of a crystalline compound of lithium gallate containing magnesium, optionally containing aluminum, and having manganese incorporated integrally into the matrix in accordance with the following formula:

x is from 0.8 to 1.0,

x is from 0.001 to 0.07,

y is from 0.001 to 0.06, and

Z is from 0 to 0.6.

2. A luminescent material consisting essentially of a crystalline compound of lithium gallate containing magnesium, and having manganese incorporated integrally into the matrix havth l nssemn s flq go.a9 o.oo5 1.aos o.2 4

wherein Mn is divalent.

3. A luminescent material consisting essentially of a crystalline compound of lithium gallate containing magnesium, and having manganese incorporated integrally into the matrix having thefollowing composition: 7

msa moos eoos a 

2. A luminescent material consisting essentially of a crystalline compound of lithium gallate containing magnesium, and having manganese incorporated integrally into the matrix having the following composition: Mg0.89Li0.005Ga1.805Al0.2O4: 0.01Mn wherein Mn is divalent.
 3. A luminescent material consisting essentially of a crystalline compound of lithium gallate containing magnesium, and having manganese incorporated integrally into the matrix having the following composition: Mg0.89Li0.005Ga2.005O4:0.01Mn 